Various forms of implementation of thermal conductivity detectors are known from the prior art. A prevalent form of thermal conductivity detectors is a thermal conductivity detector incorporating a Wheatstone bridge. The thermal conductivity detector comprises a stainless steel block, in which two separate cavities are formed, one of which contains a sample gas to be measured whereas the other contains a reference gas. A negative temperature coefficient resistor, that is, a thermistor, the resistance value of which varies depending on the temperature, is disposed in each of the cavities. The stainless steel block enclosing the cavities serves as heat coupling means between the two cavities and is usually kept at a constant temperature level that is below the temperature of the thermistor. The heat output, that is, the temperature of the respective negative temperature coefficient resistor or thermistor inside a cavity, depends on the heat conductivity “λ” of the respective gas located in each of the cavities. If the thermistors in the two cavities are interconnected in the manner of a Wheatstone bridge, the circuit will remain balanced as long as the composition of the gas in the reference cavity or the measuring cavity is the same or remains unchanged from the initial situation. Should the conductivity of the sample gas then change, for example, due to changes in the material composition of the gas in the measuring cavity, a change in temperature of the thermistor in the measuring cavity will occur and a voltage (bridge voltage) can be tapped between the two branches of the bridge. Variables such as gas content, gas pressure, humidity, etc., can thus be determined by means of the thermal conductivity detector. Such a thermal conductivity detector is, therefore, frequently used in lab environments, for example, in situations involving climatic cabinets, such as incubators, or gas chromatographs.
In order to maintain the measuring accuracy of the thermal conductivity detector, it is essential that the composition of the gas in the reference cavity remains constant throughout the measuring process. This may be achieved by a continuous flushing of the reference cavity with reference gas. This approach, however, is relatively complicated and expensive. An easier and less expensive approach is to hermetically seal the reference cavity after filling in the reference gas, so that there can be no exchange of gas and its composition remains unchanged. However, this is difficult to achieve in practice, since, on the one hand, a permanently gas-tight seal of the cavity is difficult to attain, especially when the thermal conductivity detector is used in a high temperature range. On the other hand, foreign substances can be released during the sealing of the cavity, or later during operation of the unit, inadvertently changing the gas composition in the cavity. Therefore, a drift is frequently observed, by means of which the readings on the thermal conductivity detector will become distorted over time.
It is, therefore, an object of the present invention to provide a thermal conductivity detector which has a sealed reference cavity, is easy to manufacture, and yet functions reliably and with the least possible drift over a long period of time, even at high ambient temperatures of up to 180° C. or higher.